This invention is directed to a prosthetic material having a multilayer ingrowth matrix within well-defined pores and/or channels within the material. Each layer of the matrix is either proteinaceous or synthetic, or a combination of proteinaceous and synthetic materials. Each layer of the matrix is designed to perform a specific function, such as facilitation of ingrowth of a particular cell type or release of a particular growth factor. Instead of distinct layers, the matrix can comprise concentration gradients of the same materials. A suitable application of the prosthetic material is a vascular graft.
Vascular disease in small to medium diameter arteries adversely affects arterial wall structure. As a result, blood flow through the vessel is hindered either by total occlusion or, in the opposite extreme, an acute over dilation of the vessel (aneurysm). Such indications usually require reconstructive or bypass surgery. The most successful replacements at present are autologous grafts (arteries and veins taken from the host), but often these are too diseased or unsuitable for use as an implant. There is thus a great need for the development of a reliable, fully integrated, vascular prosthesis.
Over the last 40 years, considerable progress has been made in the development of arterial prostheses. The modem era of vascular surgery began in the early 1950""s, 40 years after Carrel and Gutherie (1906) demonstrated that autologous veins could be used to replace arteries. With the advent of antibiotics and anticoagulants in ancillary medicine, the development of vascular prostheses prospered. The reversed saphenous vein was soon considered the best artery replacement and was used successfully in femoral artery replacement by Kunlin in 1949. However, the need for a smaller prosthesis led to further research by Gross and associates involving homografts using sterilized tissue. Although early results were encouraging, the long-term results were still unsatisfactory, with the grafts often failing due to thrombosis and aneurysm.
While pioneers such as Gross et al. (1948) continued to work on hetero- and homografts, Voorhees made an important observation in 1952 that changed the direction of vascular prosthetic development. After discovering that cells grew on silk thread exposed to blood, he showed the effectiveness of synthetic textile or fabric tubes as arterial replacements. A new era of vascular surgery began and the search for the most suitable material and optimal structure for a textile graft began. Experiments, even recently, have investigated factors such as knitted or woven textiles, large or small pores, different surface finishes and crimping and external reinforcing.
Presently, the materials used for vascular implants are tanned natural vessels, textile tubes made from woven or knitted Dacron, or tubes made from expanded polytetrafluoroethylene (e-PTFE). These grafts are successful for large diameter artery replacement where there is a high blood flow rate; but they have a much lower success rate in arteries with a diameter less than 6 mm. These conventional prosthetic vascular grafts do not permit unrestricted vessel ingrowth from the surrounding tissue due mostly to ingrowth spaces that are either too narrow or discontinuous. All of the present grafts eventually fail by occlusion due to thrombosis (fibrous tissue build up), or intimal hyperplasia (exuberant muscle growth at the interface between artery and graft).
Factors such as the thrombogenic nature of the graft material, surface roughness, the mechanical and haemodynamic properties of the graft and the condition of the host artery are known to influence the success of the graft. Although the reasons for failure are not fully understood, it is largely agreed that compliance mismatch between artery and graft is the predominant issue surrounding the failure of small diameter prostheses. Discontinuity in mechanical properties between the graft and artery alters the blood flow resulting in a fibrous tissue build-up leading to the complete occlusion and hence failure of the graft.
Autologous grafts, such as the saphenous vein and the internal mammary artery are still considered the best grafts for the reconstruction of small peripheral arteries, but these are often too diseased or unsuitable for use as a graft. None of the present textile grafts (e-PTFE and Dacron) have proved successful for long periods. Many approaches to graft production have been developed in an effort to create a porous polyurethane artery graft. Indeed, it has been shown that it is possible to create an initially compliant porous graft. However, the long-tern success of such grafts remains to be proven. It has become apparent that the current methods of graft construction are ineffectual and a new approach is necessary.
It is evident that the present small diameter grafts do not provide an acceptable long-term patency. Although the causes for failure are not immediately clear, it is apparent that none of the previous prostheses have the same structure as an artery or behave mechanically as an artery does. The focus of graft xe2x80x9chealingxe2x80x9d has traditionally been to achieve endothelialization. Until now, research has concentrated on developing a prosthetic material which facilitates transmural angiogenesis. However, xe2x80x9chealingxe2x80x9d appears to encompass more than endothelialization and, therefore, the focus should extend beyond the stimulation of only angiogenesis. Full integration of a vascular prosthesis involves not only endothelial cell migration and proliferation leading to a functional endothelium, but also the establishment of a functional neomedia. This would require the ingrowth of additional cell types, specifically smooth muscle cells. Furthermore, current designs of prosthetic material typically prioritize ingrowth of one cell type over another.
The present invention is directed to a prosthetic material. More particularly, the material comprises a multilayer ingrowth matrix within well-defined porosity. The matrix consists of either proteinaceous or synthetic layers or a combination of proteinaceous and synthetic layers. Each layer is designed to achieve a specific function, such that angiogenesis/endothelial ingrowth can be stimulated within one layer while smooth muscle cell ingrowth is simultaneously stimulated in a second layer, for example.
The well-defined porosity is in the form of either helically oriented, interconnected transmural ingrowth channels, or a porous wall structure containing uniformly shaped pores (i.e. voids) in a very narrow size range, or a combination of channels and pores. This invention allows for uninterrupted ingrowth of connective tissue into walls of the synthetic graft prosthesis. The problem of compliance mismatch encountered with conventional grafts is also addressed by matching mechanical properties of the graft with mechanical properties of a host vessel. These mechanical properties include smoothness, elasticity and structural integrity.
With the foregoing in mind, it is a feature and advantage of the invention to provide a prosthetic material having a multilayer ingrowth matrix within well-defined porosity.
It is another feature and advantage of the invention to provide a prosthetic material having a multilayer ingrowth matrix wherein each layer is designed to perform a specific function.
It is a further feature and advantage of the invention to provide a prosthetic material that has a surface pacifying coating and ingrowth layers within well-defined porosity for the ingrowth of specific cells, including smooth muscle cells and endothelial cells.